Professor Bob Borsley emails me:
It's extraordinary how people still claim that Latin and Classical Greek promote logical thinking without any explanation as to why these languages in particular have this property. I assume it is the complex morphology that is supposed to be important. There are two obvious points in response to this. First, there are languages with more morphology than Latin and Classical Greek, e.g. Finnish and Turkish. So if morphology is good for the brain, you ought to go for one of these (or maybe some Australian or Amerindian language which has even more morphology). Second, languages which do not have complex morphology have complexities elsewhere. A nice example is English. The 1800 pages of this book are not about nothing.What is true, I think, is that studying language in a systematic way can be just as good for developing the ability to think as studying any other complex phenomenon.